1. Field of the Invention
The present invention relates to a filter, an exposure apparatus, and a method of manufacturing a device.
2. Description of the Related Art
A plasma light source such as a laser-produced plasma light source (LPP) or a discharge-produced plasma light source (DPP) is used as an EUV (Extreme Ultra-Violet) light source. The LPP, for example, irradiates a target material placed in a vacuum chamber with high-intensity pulsed laser light to generate a high-temperature plasma so as to use EUV light which has a wavelength of, for example, about 13.5 nm and is emitted by the target material. A metal or an inert gas, for example, is used as the target material. To increase the average intensity of the EUV light emitted by the target, the pulsed laser can have as high a repetition frequency as possible, and is therefore normally operated at a repetition frequency of several kilohertz. Also, a condensing mirror is provided to efficiently use the EUV light emitted by the target. A multilayer mirror formed by alternately stacking 60 sets of molybdenum and silicon films on a mirror substrate, or an oblique-incidence mirror formed by coating a metal on a mirror substrate, for example, is used as the condensing mirror. An illumination optical system is formed by, for example, pluralities of multilayer mirrors, oblique-incidence mirrors, and reflective integrators. The reflective integrator serves to uniformly illuminate a mask at a predetermined numerical aperture. The reflective integrator receives collimated EUV light to form a secondary light source at a position defined by its focal length f.
EUV light supplied from an illumination system is reflected by a reticle serving as an original, is reduced to ¼ by a projection optical system formed by six to eight multilayer mirrors, and is guided to a wafer coated with a resist. The reticle and the wafer are held by a reticle stage and a wafer stage, respectively, and are synchronously scanned at a speed ratio proportional to the reduction magnification while being precisely aligned by an alignment optical system and precisely focused by a focus detecting optical system. In this way, an operation of synchronously scanning the reticle and the wafer while a reduction projection image of the reticle is formed on the wafer (step-and-scan) is repeated. The transfer pattern of the reticle is thus transferred onto the entire surface of the wafer.
A plasma light source emits not only desired EUV light but also OoB (Out of Band) light beams in the infrared to EUV range. These OoB light beams become a flare, which reduces the contrast of the light on the wafer and thermally expands the wafer. Also, these OoB light beams increase the heat load of a mirror, thus posing a serious problem especially in a mirror of a projection system.
Japanese Patent Laid-Open No. 2006-191090 and Wouter A. Soer, “Grid Spectral purity filters for suppression of infrared radiation in laser-produced plasma EUV sources”, Proc. of SPIE Vol. 72712Y-8 (2009), disclose methods of placing a spectral purity filter in the optical path to selectively transmit only desired EUV light so as to eliminate OoB light. In the technique described by Wouter A. Soer, the filter is provided with an opening having a size smaller than the wavelength of OoB light to transmit only EUV light and reflect OoB light (infrared light).
However, because the conventional filter reflects OoB light (infrared light) toward a mirror on the upstream side, the reflected light heats members such as a mirror and a chamber. Also, because the OoB light passes through an optical path that does not conform to a design optical path, infrared light that becomes stray light not only passes through the illumination system but also reaches even an optical element of the projection optical system and the surface of the wafer, thus heating these members.